Vibratory frequency standard



- Oct. 28, 1941. H. E. WARREN VIBRATORY FREQUENCY STANDARD Filed July 7, 1939 Fig.3.

40 .50 PERCENT NICKEL PERCENT IV/C/FEA Patented Oct. 28, 1941 UNITED STATES PATENT OFFICE VIBR ATOR-Y FREQUENCY STANDARD Application July 7, 1939, Serial No. 283,205

6 Claims.

My invention relates to vibratory frequency, or time, standard devices and particularly to such devices for use in controlling with great precision systems of clocks, power stations frequency, the movement of large telescopes, or for like uses.

This application is a continuation in part of my application, Serial No. 210,492, filed May 27, 1938.

In the above-mentioned prior application I have described a device for producing alternating current of controlled frequency by means of a vibrating wire or string tensioned by a weight, the wire being maintained in transverse vibration by electromagnetic means. The wire is described as made of a. plurality of materials, for the purpose of reducing or limiting temperature effects. In my present application a simplified form of such device is disclosed wherein the reduction or elimination of temperature effect may be obtained by the employment of a vibrating wire formed of a single material only. Further, in my present application a novel means is disclosed for compensating the effect of temperature changes upon the rate of vibration of the device.

The novel features which are considered to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing wherein Fig. 1 is a representation, semidiagrammatic, of a frequency standard device in accordance with my invention, Figs. 2 and 3 show performance curves of certain materials suitable for the wire element of the device, and Fig. 4 is a representation of a frequency standard device embodying modifications of my invention.

In Fig. 1 the numeral I designates a vibratory wire element to which isconnected a permanent magnet II. In proximity to the opposite ends of magnet H a pickup coil 12 and a driving coil 13 are provided connected to a suitable vacuum tube circuit (not shown.) The upper end of the vibratory wire element It) is connected to a rigid support l4 and gravity tension upon the wire is provided by a weight i connected through a bow-shaped spring means Hi to. the lower end of the wire. The entire vertical wire consists of a single material preferably having a low temperature coefficient and also a, low coefficient of elastic modulus. A material answering these requirements may be found in the group of nickel Invar or Nilvar, to about 45%.

steel alloys where the nickel content varies from 36%, in the substance frequently designated as The variations in the temperature coefflcient of the modulus of elasticity,

are shown in Fig. 2 which is reproduced from Circular 58 of the Bureau of Standards, page 22. The variation of the mean temperature coefllcient of linear expansion,

is shown in Fig. 3 which is plotted from the data shown in Table 14 of the above circular.

The rate of vibration of a stretched wire as shown in Fig. 1 will depend in part upon the amount of tensioning weight and in part upon the elastic stiffness in a transverse direction of the wire. Changes due to temperature may 91-- feet both the length of the wire and its modulus of elasticity. If the wire becomes longer its rate of vibration will become lower. If its elastic modulus becomes higher due to temperature changes its rate of vibration will also become higher. It will be observed from the curves of Figs. 2 and 3 that some of the alloys in this particular group have the peculiar characteristic of becoming longer with increasing temperature and also having the elastic modulus increase at the same time. Therefore, it is possible to select and proportion a vibrating wire of the type shown in Fig. 1 wherein the effect of increasing temperature on the modulus of elasticity will exactly offset the temperature effect on the length of the wire insofar as these two factors influence the time rate. In order to obtain this desirable result the wire is so proportioned that the change in its transverse stiffness will alter the rate in one direction as much as the change in the length of the wire alters the rate in the other direction. This involves the length, diameter, and loading of the wire, and it is possible to determine by experiment with a given alloy for a given frequency value the length and diameter which will insure an extremely low overall temperature coeflicient of time rate.

Another desirable means for compensating for temperature effect upon the time rate is illustrated in Fig. 4 wherein the device is similar to that shown in Fig. 1 comprising a vibrating wire, similar to the vibrating wire ID of Fig. 1, tensioned axially by a weight and formed of a material hav ng a low temperature coefficient oflength expansion and a low coefficient of elastic modulus, substantial compensation for changes in rate of vibration of the vibrating wire due to temperature change being thereby accomplished, as explained in connection with Fig. 1. The vibrating wire, however, need not have the exact proportions outlined in connection with Fig. 1 above but may have a moderately large temperature coefiicient of vibration rate either plus or minus. This plus or minus temperature coemcient of vibration rate is relatively small compared to that which would be present if no compensation were provided for in the vibrating wire itself. Nevertheless, extra compensation is required because of the presence 01 this relatively small temperature coefilcient, in addition to that provided by the weight tensioned wire itself. This extra compensation for temperature variation is then effected by the use of a supplementary spring H which supports a relatively small portion of the tensioning weight i8. Spring I! must be formed of a material which has a temperature coefficient of the elastic modulus of a proper sign to offset the temperature coetncient of rate of the weight-tensioned vibrating wire I9 including its elastic coupling bow 20. It the tendency of the vibrating wire i9 is to vibrate more slowly as the temperature increases, then the spring I! must become weaker as the temperature increases so as to support a smaller portion of the weight i8.

The relationship above stated in connection with Fig. 4 is readily computed after determining the change in rate per degree for a given temperature change when the spring I! does not form a part of the system. For example if the device loses a second per day for each degree Fahrenheit increase in temperature, the addition to the weight l8 necessary to compensate will be the amount in grams necessary to produce the same increase in daily rate for one degree multiplied by the number of degrees rise. Since there are 86,400 seconds in a day and the rate is substantially proportional to the square root of the weight, therefore for each second per day approximately 4 part of the weight must be added. Now if the spring ll be so chosen that its total lifting eiiect varies by the same amount for the same change in temperature and it becomes weaker with increasing temperature, the net result will be a loss of lifting effect and an increase of the desired amount in the tensioning force acting upon the wire.

Inasmuch as the lifting effect of the spring I! is adjustable at will by the knurled nut 2! the compensating effect of the spring IT may be readily increased or diminished as desired. However, the rate must be kept correct by adding or subtracting small units of weight, as weight 22, to offset changes which are made in the lifting effect of the spring. Obviously the adjustable nut may also be used for small rate adjustments after the tension of the spring is made approximately correct. An equivalent efiect o! the spring may be obtained in other ways. For example, a spring may be arranged to add pressure to the tensioning weight It in which case the spring may consist of material having a temperature coefficient of the modulus of elasticity in the reverse direction, or instead of a spring a magnetic air gap between temperature sensitive materials may be arranged to add to or subtract from the tensioning weight [8 and thus produce the same effect as the spring.

For a third method of reducing temperature effect on the vibrating wire I have discovered that a material like tungsten may be employed for the vibrating wire which has a low temperature coefllcient or length expansion and which has such a very high tensile strength as to permit the use of extremely small diameters to support a comparatively heavy tensioning weight. Under these conditions the effect or the elastic transverse stillness of the wire may become practically negligible so that its temperature modulus of elasticity is no longer troublesome. In this latter manner it is possible to construct a time standard which will have a rate variation 01 less than one tenth oi a second per day per degree Fahrenheit change in temperature without other compensating means such as before described. The use or a drawn quartz fibre which has a temperature coeflicient or length expansion even smaller than the tungsten may also be desirable.

The means for the reduction of temperature disturbing effects as thus Iar set forth herein may be summarized as follows:

1. The use or a material for a vibrator wire which has a temperature coetllcient or length expansion of opposite sign to its temperature coefllcient oi the modulus of elasticity, the length, diameter, and loading of the vibrator element being so proportioned that the effect 0t these opposite temperature coeflicients will neutralize each other so far as the rate of vibration is concerned.

2. The use of a vibrator element which may have a small temperature coemcient or its rate of vibration, this eflect being compensated by an increase or decrease of the tensioning weight brought about by an elastic member which has a controllable temperature influence upon the amount of the tensioning weight.

3. The use of a vibrating member having such proportions of length, diameter, and loading that the effect of its temperature coefficient 01 the elastic modulus is substantially negligible and its temperature coeflicient of length expansion very small.

In addition to the elimination or great reduction of the eflfect of temperature upon the vibration rate or the vibrator by the methods and means outlined hereinabove, I have further provided means in accordance with my present invention whereby a marked improvement in the compensation for variations in amplitude of the vibrating wire is produced. In my above mentioned prior application, Serial No. 210,492, I have pointed out the advantages of an elastic coupling between the weight and the vibrator wire, and also that the system as a whole must be so proportioned as to avoid troublesome resonance of different modes of vibration, especially with respect to transverse and vertical vibration. One form of such compound vibration which is especially objectionable results if normal vertical frequency of vibration or the system constituted by the tensioning weight and the wire is the first harmonic of the transverse vibration of the wire. because such vertical vibration will then absorb large amounts of energy from the transversely vibrating system.

I have discovered that ii an elastic coupling of the form shown in Fig. 4 consists of a spring bow, as 20, preferably made of elastic material, such as Elinvar, which has a very low temperature coeflicient of elastic modulus is tuned by means of an adjustable weight 23 so that its normal period of vibration considered separately from the vibrating wire is twice the normal frequency of the wire, that the elastic bow 20 together with weight 23 will then act as a wave filter which will tend to reject energy from the vibrating wire, or in electrical terms serve as a high impedance to the transfer of such energy. This is very similar to a parallel condenser and inductance combination tuned to act as a high impedance in an alternating current circuit.

The impedance of the above described tuned bow 20 is so effective that the tensioning weight l8 remains practically motionless regardless of the amplitude of vibration of the wire. Consequently the amount of electrical energy required to drive the wire becomes much less and the eifect of changes of amplitude of the wire makes no appreciable change in its rate of vibration. The gain in this respect is so great that a curve showing the change of vibration rate with variation in amplitude of vibration, becomes a straight horizontal line, on the usual scale which is used in producing that curve. It is possible in this manner to reduce the error due to changes in amplitude to less than a second a day for a varia tion in amplitude of 100%.

My invention has been described herein in particular embodiments for purposes of illustration, It is to be understood, however, that the inven tion is susceptible of various changes and modi fications and that by the appended claims I intend to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a frequency standard apparatus, a vibratory member, a weight to tension said member axially connected thereto at one end thereof, means to vibrate said member operatively associ ated therewith at a point intermediate the ends thereof, and an elastic member coacting with said weight to tension said vibratory member, said elastic member having a temperature costlicient of its modulus of elasticity of such value as to tend to neutralize changes in rate of vibration of said vibratory member produced by tempera ture variation of said vibratory member.

2. In a frequency standard apparatus, a vibratory member, a weight to tension said member axially connected thereto at one end thereof, means to vibrate said member operatively associated therewith at a point intermediate the ends thereof, and a temperature responsive element coacting with said weight in producing tension in said member, said element having a temperature coefiicient of its modulus of elasticity of such val no as to tend to neutralize changes in rate of vibration of said member produced by temperature variation of said member.

3. In a frequency standard apparatus, a vibratory string, means to tension said string axially connected thereto at an end thereof, a driving means operatively associated with said string at a point intermediate the ends thereof, said string being adapted to be vibrated transversely by said driving means, and an elastic coupling associated with one end of said string and tuned to vibrate at twice the normal transverse frequency of vibration of said string.

4. In a frequency standard apparatus, a vibratory string, means to tension said string axially connected thereto at one end thereof, a driving means operatively associated with said string at a point intermediate the ends thereof, and an elastic coupling associated with said string and separately tuned to provide a high impedance to the tramfer of vibratory energy in an axial direction from said string.

5. in a frequency standard apparatus, a vibratory member, a support for one end thereof, a weight to tension said member axially connected thereto the other end thereof, and means to vibrate said. member between said support and said weight, means being operatively associated therewith. at a point intermediate the ends thereof, said vibratory member having a constant rate of vibration at a predetermined temperature thereof, said vibratory member having a temperature coefficient of length expansion and a temperature coefficient of its elastic modulus and which are of such values as to reduce in amount changes in its rate of vibration produced by temperature variation of said member whereby deviation of the vibration rate of said vibratory memher from said constant rate due to deviation of the temperature of said vibratory member from said predetermined temperature is reduced.

6 In a frequency standard apparatus, a vibratory member, a support for one end thereof, a weight to tension said member axially connected thereto at the other end thereof, means to vibrate said member between said support and said weight, said means being operatively associated therewith at a point intermediate the ends thereof said vibratory member having a constant rate of vibration at a predetermined temperature thereof, said vibratory member having a temperature coefficient of length expansion and a temperature coefficient of its elastic modulus which have opposite effects upon the rate of vibration and which are of such values as to tend to neutralize changes in its rate of vibration produced by temperature variation of said mem ber, whereby deviation of the vibration rate of said vibratory member from said constant rate due to deviation of the temperature of said vibratory member from said predetermined temperature is prevented.

HENRY E. WARREN. 

